Certain applications can involve the use of multiple internal combustion engines that are harnessed together to drive one or more loads. One such application can be the generation of electrical power for an electrical load using multiple generator sets (known as “gensets”) with each genset including in combination an engine and an electrical generator or alternator. Marine vessels are another application that can include multiple engines harnessed together to drive one or more primary loads (e.g., propellers) and various auxiliary loads (e.g., HVAC, lighting, pumps, etc.). The engines can be mechanically connected to the loads or electrically connected to the loads by way of generators. In some applications, the loads of a vessel can be driven both mechanically and electrically in a hybrid arrangement.
In typical multi-engine applications, all engines are simultaneously operated to produce about the same amount of power. For example, a particular marine vessel may have four identical engines each capable of producing about 5,000 kW. And during operation, all of the engines may be operated at the same level (e.g., at about 20% capacity) to evenly distribute the loads (e.g., to evenly distribute a 4,000 kW load). If different engines have different output capacities than others in the group of engines, the engines may be operated synchronously based in proportion on their individual rated capacities for power output in a manner sometimes referred to a symmetric load sharing or symmetric loading. In symmetric loading, each engine is operated to output power according to its relative capacity in proportion to the total capacity of the combined engines. Hence, the engines are all being operated at the same percentage of their individual, relative capacity, and theoretically should be subjected to the same level of stress and wear, even though some engines may be producing a larger absolute output than other engines.
While these load sharing strategies help ensure that each genset of the plurality is operated within its capacity and capabilities, they do not take into account other considerations associated with the operation of the engines, such as the total emissions produced by the engines. Internal combustion engines exhaust a complex mixture of air pollutants. These air pollutants are generally composed of particulates and gaseous compounds including nitrogen oxides (commonly referred to as NOx) among others. Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amounts of particulates and NOx emitted into the atmosphere by an engine may be regulated depending on the type of the engine, size of the engine, class of the engine, and the like as well as the location in which the engine is operating. For example, the engines operating on a marine vessel may be subject to different regulations depending on whether the vessel is in port or at sea. Using a strict symmetric load sharing strategy for a multi-engine application may result in higher than desired total emissions across the plurality of engines.
Moreover, in order to comply with the regulation of particulates and NOX, some engine manufacturers have implemented a strategy called selective catalytic reduction (SCR), which is a process where a reagent known as diesel exhaust fluid (DEF), most commonly urea, or a water/urea solution, is selectively injected into the exhaust gas stream of an engine and absorbed onto a downstream substrate in order to reduce the amount of NOX in the exhaust gases. However, the efficiency and cost of the operation of a SCR system can vary depending upon the load condition of the engine. A strict load sharing strategy for multi-engine applications does not take into account SCR system operating conditions.
U.S. Publication No. 2005/0282285 (“the '285 publication”) discloses a strategy for controlling NOX emissions in an SCR system associated with an internal combustion engine. However, the '285 publication does not address the total emissions produced by a plurality of engines harnessed together to drive a load.